Dual track stencil/screen printer

ABSTRACT

A dual track stencil system includes a first set of rails and a second set of rails, each of the rails being independently controlled for allowing for independent movement of circuit boards through the stencil system for processing. The stencil system includes a working area, at which the circuit boards are stenciled. A stencil, having two patterns, each aligned with a respective working area of each track is suspended above the dual track system. A solder dispensing unit including a squeegee arm is suspended above the stencil, parallel to and centered between the two tracks. During operation, a circuit board enters the working area, is raised to engage with the stencil, and the squeegee arm traverses that portion of the stencil pattern currently engaging the board, thereby dispensing solder through the pattern and stenciling the board. The stencil system operates to process two boards on one track before advancing to the second track for processing another pair of boards. The rails of the dual track system are advantageously moveable to allow for variable width boards or operation of the system in single track mode. The stencil system design may be adapted to make the dual track system a recirculating track, capable of supporting pallets for transport and printing of delicate materials. In addition, the stencil system design may be adapted for processing boards in parallel, where the boards are received simultaneously or staggered, placed at a stencil position, and printed with one stroke of the squeegee head.

FIELD OF THE INVENTION

This invention relates generally to the field of circuit board manufacturing and more specifically to a method and apparatus for improving the performance of circuit board stenciling operations.

BACKGROUND OF THE INVENTION

As it is known in the art, the success of circuit board manufacturers is measured in terms of the number of defect-free products they are able to supply in a given time period. Unfortunately, the achievement of desired productivity levels is hampered by the accuracy requirements inherent in providing such defect-free products. The accuracy requirements become even more difficult to attain as technological advances command smaller component sizes and more pins per package fitting on a circuit board with very closely spaced pads. Defective boards may be either rejected outright or reworked, both of which costs time and increases expense.

For example, the typical circuit board manufacturing process includes three basic operations, each of which is usually performed by an independent machine. These operations include stenciling of a pattern of solder onto the pads of an incoming circuit board, placement of components on the pads of the board, and heating the board to establish the integrity of the contact between the solder and the board, the pins of the components, and the pads.

The stenciling and the placement operations must be performed in a precise manner to ensure that a defect-free product is provided. The stenciling operation is typically performed by a stencil machine. Boards that are fed into the stencil machine have a pattern of pads or other, usually conductive surface areas onto which solder will be deposited. In addition, one or more small holes, or marks, called fiducials, are provided on the board for alignment purposes. The stencil machine also typically includes a stencil (or screen) having a pattern of apertures etched through the stencil that matches the expected solder pattern to be placed on the circuit board.

When the circuit board is fed into the stencil machine, it first must be aligned with the stencil to make sure that the circuit board is appropriately aligned with the apertures of the stencil. One apparatus for performing this task is an optical alignment system that is introduced between the circuit board and the stencil for aligning the fiducials of the board with one or more fiducials on the stencil. For example, optical alignment systems provided by MPM™ Corporation, such as those described in U.S. Pat. No. 5,060,063, issued Oct. 21, 1991, by Freeman and in U.S. Pat. Re. 34,615 issued Jan. 31, 1992 by Freeman, each of which is incorporated herein by reference, may alternatively be used.

When the board has been aligned with the stencil, it is raised to the stencil, solder is dispensed on the stencil, and a wiper blade (or squeegee) traverses the stencil to dispense the solder through the apertures of the stencil onto the board. After stenciling is complete, the board is lowered and forwarded to a pick and place machine that places electrical components at specific locations on the board.

The pick and place machine provides the components that are required by the board. Components are placed at the appropriate locations (the location of solder deposition performed by the stencil machine) and with the proper orientation on the circuit board. Because of the number of pins of a component and the proximity between the pins, it is essential that the pick and place machine operate to precise standards to ensure the correct placement of the component upon the soldered connections. Thus the pick and place machine must also perform alignment checks before operation.

The next step in the process is to forward the circuit board having the components laid upon the stenciled solder to a reflow machine. The reflow machine heats the existing solder to cause the solder to form tight connections with the pins of the components that were placed by the pick and place machine and the pads or other surfaces of the board.

In the systems described above, the accuracy required in the printing of solder on circuit boards and in the placement of components on the printed solder leads to the use of alignment procedures for the circuit board which cause delays in the manufacturing process. In addition, there is an added element of delay, referred to as `dwell time` at the pick and place machine as the pick and place machine waits to receive a board from the stenciling machine. Each of these delays reduces the overall performance and productivity of the manufacturing process. It would be desirable to develop a method and apparatus for increasing the throughput of the circuit board manufacturing process.

SUMMARY OF THE INVENTION

According to one aspect of the invention, a stenciling device for use in a circuit board manufacturing process includes a plurality of sets of rails, each of the plurality of sets of rails supporting a corresponding plurality of circuit boards as the circuit boards propagate through the stenciling system. With such an arrangement, in a preferred embodiment, a dual track system may be provided that allows for the reduction of dwell time in the circuit board manufacturing process.

According to another aspect of the invention, a method for providing processing of circuit boards in a stenciling system having first and second tracks, the stenciling system comprising a stencil having two patterns, one associated with each track, and a solder dispensing mechanism, suspended above and between the two patterns of the stencil includes the steps of: receiving a first board on the first of the tracks, engaging the board, stencil, and solder dispensing unit, stroking the solder dispensing unit from a first position between the patterns to an outer position of the one of the tracks receiving the board, disengaging the first board, advancing the first of the tracks to receive a second board, engaging the second board, stencil and solder dispensing unit, stroking the solder dispensing unit from the output position of the first of the tracks to the first position, and repeating the above steps using the second of the two tracks.

With such an arrangement, a processing sequence is provided that allows for existing stenciling components to be used in a dual track system, thereby minimizing the extent of additional hardware by eliminating the need for a second stencil apparatus and minimizing the overall cost of the system.

According to another aspect of the invention, a dual track stenciling system having shortened rails is described. With such an arrangement, the stencil machine may be used at circuit board manufacturing processes having existing pre- and post-print queue stations on loading and unloading equipment.

According to a further aspect of the invention, a dual track stenciling system for stenciling of palletized material includes a first set of rails for receiving boards from a loader/unloader, a second set of rails for returning boards to the loader/unloader, a shuttle device for transporting boards from the first set of rails to the second set of rails and a plurality of palettes, for supporting boards as they are transported through the stenciling system. With such an arrangement, the dual track stencil machine may be used for printing of palletized materials.

According to a further aspect of the invention, a dual track stenciling comprises a pair of tracks, each track for forwarding in parallel a circuit board to a work nest of the stencil machine for processing.

According to one aspect of the invention, a stenciling system for use in circuit board manufacturing includes a plurality of sets of parallel rails mounted perpendicular to a plurality of spacing tracks, each of the sets of rails for supporting a corresponding plurality of circuit boards as the circuit boards are moved through said stenciling system, wherein each rail of is separately controllable to be moved in along said spacing tracks. With such an arrangement, a stenciling system capable of stenciling varying size boards in one process is provided. In addition, the use of moveable rails allows for increased compatibility between the stenciling system and a variety of pick and place machines provided by other vendors, which may have different fixed location rail orientations and changing Printed Circuit Board (PCB) sizes.

According to another aspect of the invention, the stenciling system is capable of operating in either multi-track mode, wherein all of the tracks are used for stenciling circuit boards, or in single track mode, wherein only one of the plurality of tracks is used for stenciling circuit boards. By providing a separate mode of operation, large width circuit boards, requiring stenciling using the majority of the working area of the stenciling machine, can be accommodated without requiring manual changes to the machine.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is the flow diagram illustrating the discrete steps used in a circuit board manufacturing process according to the present invention;

FIG. 2 is a diagram of a stenciling machine for use in the circuit board manufacturing process of FIG. 1;

FIG. 3 is a block diagram illustrating the control structure of the stenciling machine of FIG. 2;

FIG. 4 is a side view in part of the stenciling machine of FIG. 2, illustrating one manner of operating that stenciling machine;

FIG. 5 is a diagram of the rail system of the stencil system of FIG. 2, illustrating the placement of the dual stencil over the dual rails of the stencil system;

FIGS. 6a-6d are block diagrams illustrating the sequence of handling of circuit boards through the stencil system of FIG. 2;

FIG. 7 is a diagram illustrating the motor control and rails of the stenciling system of FIG. 2;

FIG. 8 is a diagram illustrating the arrangement of rails in the operation of the stenciling system in single track mode.

FIG. 9 is a diagram illustrating the position of rails in a single track mode;

FIG. 10 is a diagram illustrating the position of rails in a dual track mode;

FIG. 11 is a flow diagram illustrating a reboot/reset process;

FIG. 12 is a diagram of another embodiment of the dual rail system, where the dual rail system has shortened rails;

FIG. 13 is a diagram of another embodiment of a dual rail system that provides all material handling on a single side of the system.

FIG. 14 is a diagram of another embodiment of a dual rail system that provides all material handling on a single side of the system; and

FIG. 15 is a diagram of a dual rail system having a mechanical fixturing device that allows printing on two boards simultaneously.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, an apparatus for providing improved performance for circuit board manufacturing includes a dual track rail system 11 for providing a pair of circuit boards 12a and 12b to the stenciling machine 14. The dual track nature of the system is incorporated at the pick and place machine, thus allowing for two boards to be independently forwarded to the pick and place machine 16. Each of the tracks extends to provide a dual input path to a reflow machine 18. Completed circuit boards 20a and 20b exit from the reflow machine 18. The interface between the stencil machine 14, the pick and place machine 16 and the reflow machine is controlled according to protocols set forth by the Surface Mount Equipment Manufacturing Association (SMEMA).

Providing a dual track system allows for the performance of the circuit board manufacturing process to be improved since the dwell time formerly incurred at the pick and place machine may be eliminated. This is accomplished by providing a second board to a second track of the pick and place machine while the pick and place machine is operating on a first board on the first track. When the pick and place machine completes operation on the first board, it simply moves to the second track without having to wait for a second board to be transferred into its work area.

It should be noted that the dual track system of the present invention has the flexibility of operating as a parallel processing machine, by forwarding two boards through on the track and processing them simultaneously, or alternatively operates on only one track at a time. In addition, the dual track system may be configured to work with robotic loaders, transferring boards into and out of the working nest on only one side of the stencil machine, or additionally may be configured to work with pallets for handling sensitive materials, by forwarding a material on one track to be stenciled, and returning it on the other track. These embodiments will additionally be described below. Suffice it to say that all of these embodiments may be implemented without adding a large amount of additional hardware to prior art stenciling systems. In addition, it will become apparent that the dual rail stenciling system may be optimally used for manufacturing processes where either a top and bottom side of a board are needed to be printed, where a mother and daughter board need to be printed, or for faster processing of one board.

Referring now to FIG. 2, one embodiment of a stenciling device 30 according to the present invention is shown. The stencil device 30 includes rails 22, 24, 26 and 28. The rails provide movement along the x-axis 25 of circuit boards as they are input to the stencil machine 30. In this embodiment of the invention, each pair of rails, such as rails 22 and 24, form one track. Each rail of each track further includes a conveyor belt 54a, 54b, 54c, and 54d, shown in FIG. 5, on which the circuit board rests as it is propelled through the stencil machine. Each of the tracks are independently controlled for allowing boards on different tracks to be moved to different positions simultaneously.

Area 37 is the "work nest" of the stenciling device 30, which includes a Z-tower (not shown here) for providing movement of the board along the z axis 29. The work nest is the area in the machine at which the stenciling of the board occurs.

A stencil support 38 is included on either side of the rails. The stencil support 38 allows a stencil (not shown) to be inserted in the y-axis 27 and secured above the rails. More details on the stencil will be provided later herein. It should be noted that although herein the term `stencil` is used, a screen may also be used.

A squeegee arm 35 is shown to include a pivoting squeegee head 32, a squeegee motor 33 and a paste dispenser 34. Although the discussion here will refer to the distribution of solder paste, it should be understood that ink, glue, or other material may also be substituted during material deposition. The squeegee head 32 comprises a pair of blades manufactured from metal, polyurethane or stainless steel, or other such material, and tilts at an angle that allows the solder paste to be dispersed across the stencil. One suitable squeegee head is the Pro-head squeegee available from MPM Corporation of Franklin, Mass. The squeegee arm moves in the y-axis 27 over the stencil and rails. The paste dispenser 34 supplies solder paste to the pivoting squeegee head, generally on an x-axis 25 in line with the rails. It should be noted that while the squeegee head herein is described as having blades, in fact the squeegee head may be any device capable of distributing solder paste through apertures of a stencil.

A vacuum unit 36 may also provided, with separate vacuum hoses (not shown) allocated to each track of the dual track system. The vacuum unit 36 provides suction on the underside of a board in the work nest to maintain the board's position. In alternate embodiments of the present invention, side snubbers or gripping fingers that hold the circuit board (or some other substrate that is to be printed upon) in place during the printing operation may be used in place of the vacuum unit.

The operation of the squeegee arm, the paste dispenser, the pivoting squeegee head, the dual tracks and the vacuum are all controlled via system control software 39 executing on a computer system 21 coupled to the stencil machine. Referring now to FIG. 3, a block diagram illustrating the control structure of the stencil machine 30 is provided. In the described embodiment, circuit boards are loaded into the stencil machine from a loader (not shown). A sensor 38, receives indications from the loader of the availability of a board on each of the tracks. In addition, the sensor 38 receives indications from the pick & place machine 16, indicating whether or not the pick and place machine is available to receive boards from the stenciling machine on a given track. The loader, stencil machine, pick and place machine and reflow machine communicate using a SMEMA handshake protocol.

The sensor 38 feeds the status received from the loader and the pick and place machine to the computer system 21 (FIG. 2). The computer system may also receive inputs from a user of the system, indicating, for example, the size of the circuit boards. The computer system 21 is coupled to moveable components of the stencil machine. Depending upon the complexity of the movement of a component, associated with the component is a control device for controlling the operation of an attached motor. The control device is either a complex control device, such as a microprocessor, or a simple programmable logic device, depending upon the complexity of the movement that must be made by the component. The computer system 21, through the system control software 39, issues control signals to each of the controllers, such as track 1 controller 39a, track 2 controller 39b, squeegee/paste controller 39c, rail 1 motor controller 39d, rail 2 motor controller 39e and rail 3 motor controller 39f. The operation of each of these components will be more thoroughly described later herein.

Referring now to FIG. 4, a general description of the operation of one track of the stencil machine will now be provided. Although only one track is described here, the same operation occurs at both tracks when a circuit board at that track is being stenciled. As shown in FIG. 4, the circuit board 12 is forwarded into the stencil machine via conveyor belts, such as conveyor belt 41 of the rail 22, and placed at the proper position above the Z-tower 42. The Z-tower 42 includes a number of supports 42a and 42b as well as advantageously a center support (not shown) to preclude bending of the circuit board during the stenciling process. When the circuit board is slid into the Z-tower by conveyor belt on each of the rails, the vacuum system 36 is turned on to provide a gentle downward pressure on the circuit board 12 for holding the circuit board in position. A porting plate 43 incorporated within a top plate 43a of the Z-tower connects the vacuum system to the air space of the Z-tower. The porting plate 43 may be formed from multiple pieces of stainless steel or other rigid material, and includes a number of apertures through which air flow is permitted. Forming the porting plate from multiple pieces of stainless steel allows for the size of the plate, and consequently the location of the apertures to be altered by simply substituting one of the pieces. The apertures of the porting plate are positioned such that the downward pressure of the vacuum is directed at locations of the circuit board that will center the downward pressure of the vacuum on the board.

As described previously, a stencil 40, slid into the stencil support 38, remains fixed in that location until replaced by another stencil.

When the circuit board is ready to be stenciled, the Z-tower 42 elevates the board on the z-axis above the conveyor belt 41 to contact the board and the stencil. The paste dispenser 34 dispenses a line of solder paste at a location on the stencil adjacent to a pattern on the stencil. Once the solder paste has been placed on the stencil, the pivoting squeegee 32 is stroked across the stencil in the y-axis direction 27 to displace the solder such that the solder is distributed through the apertures of the stencil and onto the circuit board. After the y-axis motion of the squeegee has completed, the circuit board stenciling is complete, and the Z-tower 42 descends in the z-axis to its original position. When the Z-tower has descended to its original location, the circuit board will again rest upon the conveyor belt 41 and will be forwarded out of the stenciling device to the pick and place machine.

Referring now to FIG. 5, it will be appreciated that if there are two circuit boards input on dual parallel tracks, then there are two separate stencil patterns cut into stencil 40. In embodiments of the present invention, stencil 40 is formed by stamping or otherwise forming holes in desired locations on the stainless steel plate 40a. The outer edges of the stainless steel plate are coupled to a resilient polyethylene screen 40b mounted on a stiff aluminum frame. It should also be noted that other materials, such as plastics, may alternatively be used, or that a screen may be used instead of a stencil. The overall dimensions of the framed stencil may be, for example, 29"×35", although these dimensions may vary according to the requirements of the user. The two stencil patterns may either be the same or different. One advantage of the present invention is that it allows for the stenciling of both the top and bottom of a circuit board with one stencil.

In FIG. 5 a number of circuit boards 50a, 50b and 50c, are shown traversing the rails 22-28 during operation, and a number of circuit boards 52a, 52b, and 52c are shown travesing the rails 22 and 24. As mentioned previously, the circuit boards lie on conveyor belts surrounding the rails. The conveyor belt may be made of either fabric, rubber, or a similar material. The two tracks, that is track 1 including rails 22 and 24 and track 2 including rails 26 and 28, are each powered separately to allow for separate advancement of circuit boards along the tracks.

For example, referring now to FIG. 6a, two boards, board A1 and board B1 are shown on respective tracks A and B. The relative operational positions of the board shown in FIGS. 6a-6d are indicated by dashed boxes and labeled position 1, position 2 and position 3. The squeegee head 32 is shown in FIG. 6a positioned in the center of the two tracks. In the embodiment shown in FIGS. 6a-6d, the Z tower, denoted by box 55 in FIG. 6 is formed as one unit spanning both tracks. The Z-tower is raised in the z-axis whenever stenciling needs to be done on either of the tracks. In alternative embodiments of the present invention, a separate Z-tower may be provided for each track.

The stencil printing process begins by identifying which track to begin printing on. If there are no boards in the stencil printer, it will accept a board from a board loader (not shown), coupled to each track, and move both boards A1 and B1 to position 1 in the track as shown in FIG. 6a. If the loader on track A has a second board available, then the printer will convey the board A1 on the first track to position 2. Once board A1 has moved to position 2, processing can begin on board A1. As described previously with respect to FIG. 4, the processing begins by elevating board A1 using the Z-tower to bring board A1 into contact with the stencil. Solder is distributed on board A1 by traversing the squeegee head 32 along the y-axis from a position adjacent to rail 24 to a position adjacent to rail 22. While board A1 is elevated off the conveyor belt, board A2 is forwarded to position 1. When the printing on board A1 has finished, the Z-tower will descend and board A1 will again rest on the conveyor belt of track A. Board A1 will move to position 3 and board A2 will move to position 2.

During the next printing cycle, solder is printed on board A2. As described above, the Z-tower will elevate board A2 to bring it into contact with the stencil. The squeegee head 32 will traverse from the position adjacent to rail 22 back to the position adjacent to rail 24 to complete the printing process. During the period of time that A2 is elevated, board A1 may be moved from position 3 to the pick and place machine as the pick and place machine becomes available.

In an embodiment where the Z-tower is one unit spanning both tracks, board B1 is precluded from advancement along track B during the period of time that board A2 is elevated by the Z-tower. Board B1 could be moved into stencil position on the B track before board A2 is elevated, however, unless the system is designed to simultaneously process two boards (as is the case for other embodiments described hereinafter) this may cause undesirable contact between the stencil and board B1.

In an embodiment where there is a separate Z-tower for each track, the advancement of board B1 to the stenciling position may be performed at any time during the advancement or stenciling of the A2 board. Suffice it to say that, in the example shown in FIG. 6d, when board A2 has been processed and the Z-tower is lowered, board B1 may be moved into position 2. At approximately that time, board A2 may be moved into position 3.

During the cycle that board B1 has moved into position 2 and board A2 has moved into position 3, board B2 may remain in the loader. In a preferred embodiment, after board B1 has been raised by the Z tower 55 to lift board B1 off the conveyor belt, board B2 is advance to position 1 as shown in FIG. 6d.

The above sequence of events will execute continuously provided there are boards available in the loader. However, it is possible that during the print process problems will occur with one or more of the components of the circuit board manufacturing system. The computer system 21 and system control software 39 recognize these problems and adjust the operation of the stencil machine accordingly. For example, if a first board in a set of two consecutive boards is located in position 1, the stencil printer will not allow it to advance to position 2 for printing unless the loader indicates that there is a second board waiting to move into position 1. If the loader does not indicate, through known suitable means (not shown), that there is a second board waiting, the system control software indicates a loader empty situation for the corresponding track and directs the stencil machine to switch to single track printing on the other track. During single track printing, circuit board printing occurs on only one of the tracks. The system control software also modifies the operation of the movement of the squeegee for single track printing to eliminate the squeegee being stroked over stencil patterns under which no circuit board is positioned.

If a board is located in position 1 on a track and the loader indicates that another board is available for processing on the same track, control software executing in the stencil machine will not allow the board in position 1 to advance to position 2 for printing unless there is an open position on the same track of the pick and place machine. The pick and place machine and the stencil machine communicate using a handshake protocol set forth by the Surface Mount Equipment Manufacturing Association (SMEMA). If the pick and place machine indicates to the stenciling machine that there is no open position on that track, control software at the stencil machine detects this as a placement track down situation, and resorts to single track printing on the other track.

Before resorting to single track mode, the stencil printer control software examines status information received from the loader to determine if a backup problem exists on the other track. If it does, the control software interprets this as a slow down associated with outside influences, such as unavailability of boards. In such an event, the control software waits in dual track mode and proceeds printing on the first track that becomes free.

While in single track mode, the stencil printer control software monitors the information received from the loader and from the pick and place machine, and reverts back into dual track mode when possible. This change back occurs when the squeegee head is located in the center of the stencil.

If the second set of two consecutive boards is in position 3, and control software determines (in response to signals received from the pick and place machine) that it is unable to move to the pick and place machine, the stencil machine does not load another board into position 1. Rather, the stencil machine views this as a track down situation and resorts to single rack printing on the other track.

If, at a given track, one board is in position 2 and a second board is in position 3 but is unable to move to the pick and place machine, the stencil printer control software will be unable to move the printed boards out of position 2 and will resort to single track printing on the other track. In this case, the stencil printer control software will signal that manual intervention is required and begin single track printing on the other track.

In an alternative embodiment of the present invention, to avoid the need for manual intervention in the above-described situation, a pin located between the rails is raised in response to a command from the computer system. The track is then moved backwards to move the board in position 2 to position 1. The pin prevents the board at position 3 from moving back into position 2. The stencil machine can then continue operation in single track mode.

If, at a given track, one board is being printed in position 2 and the stencil printer control software determines that a second board is unable to exit the loader and enter position 1, the stencil printer control software may call for manual intervention to load the second board. Alternatively, the control software may advance the first board to position 3, return the squeegee head to the center of the stencil, and begin single track printing on the other track.

It should be noted that all of the above protocols are implemented using a system control software program that controls the motors of the conveyor belt of the rails and the positioning of the squeegee head. In addition, the system control software interacts with the adjoining components of the circuit board manufacturing system according to a SMEMA handshake protocol.

Referring back briefly to FIG. 5, the movement of the squeegee across the stencil will now be described. In one embodiment of the invention, the processing order of the boards as they are forwarded through the stencil printer is dictated by the location of the squeegee head relative to the two stencil patterns. For example, as shown in FIG. 5, the squeegee head is centered above the two patterns of the stencil when it is in its start up or "home" position. The paste dispenser places two beads of solder paste (or some other similar material) 46a and 46b adjacent to each respective pattern on the center portion of the stencil.

During operation, the squeegee head 32 moves, for example, to print on board A1 as described in FIG. 6a, from position A to position B (shown in FIG. 5). As mentioned previously, in one embodiment, the squeegee head comprises a pair of blades which are positioned over the stencil's surface. It should be noted that, in an alternative embodiment, the blades may be independently movable up and down to engage and disengage from the stencil's surface, with one of the blades being inclined or tilted along the y-axis for motion in one direction and the other of the blades being inclined or tilted along the y-axis, for motion in the other direction.

The computer system 21 causes an operation to tilt the squeegee head along the y-axis to allow a first one of the squeegee blades to contact the solder paste. The second blade is lifted off from the solder paste engaging position on the stencil's surface. In this embodiment as described, the solder paste is positioned between and travels between the blades. The squeegee head strokes the solder paste across the stencil to thereby dispense solder through the apertures of the pattern and onto board A1. After processing of board A1, the squeegee head will be in position B. At this point, there is solder remaining between the blades of the squeegee head that is to be used to process board A2.

If there is no second board on track A to be processed, squeegee head 32 is moved from position B to position A' (for processing boards on track B). This transition preferably occurs without dragging the wiper blade across the stencil, since such an operation would cause the solder to be stroked over apertures of the stencil when there was no board in position. In addition, it is undesirable to lift the blade and move to position A', since solder would remain at position B that may ultimately become dry and non-viscous, thereby compromising the integrity of the stencil. Further, if the blade is lifted, paste may drip from the blade as it is moved to position A'. Embodiments of the present invention are capable of performing such a lifting and moving operation of the blades when necessary for maintaining system performance. However, for the reasons described above, this should be minimized.

Thus, for the above reasons, it is advantageous to process the boards in the order described above with reference to FIGS. 6a-6d. In the preferred order described above, when board A2 is advanced to the stencil position, squeegee head 32 will tilt along the y-axis to move the first blade of the squeegee head off from the solder-engaging position and to enable the second blade of the squeegee head to stroke the solder from position B to position A₁ in FIG. 5. The solder paste for processing board A1 is located between position A and position A₁.

The squeegee head then lifts the blades and moves from position A₁ to position A' on track B, thereby jumping over solder line 46b and avoiding contact pressure on the center of the stencil. The squeegee head operates in a similar manner at track B, moving from position A' to B' for processing board B1, and from position B' to A₁ ' for processing board B2.

In an alternative embodiment, two independently movable squeegee heads may be disposed above the stencil 40 with two stencil patterns cut into the stencil, as shown in FIG. 5. In this alternative embodiment, the two squeegee heads, which may be constructed or described above, may each be independently stroked over a respective stencil pattern in the same directions of movement of head 32 shown in FIG. 5. The rest or "home" position of the two squeegee heads may be either between the two stencil patterns or outside both stencil patterns. In this alternative arrangement, two circuit boards may be processed or stenciled at the same time if, of course, two circuit boards are in position to be processed. Further, in this alternative arrangement, the sequence described above and shown in FIGS. 6a-6d may be modified to simultaneously deliver circuit boards to the area of the stencil machine at which processing occurs. Alignment of the two circuit boards may be performed by the optical alignment step described above or with the mechanical fixturing device described in further detail below.

In a second embodiment of the invention, the dual track system for a stencil machine described above is modified to provide movement of the rails along the y-axis. By allowing the rails to move along the y-axis, different size boards may be accommodated.

The selection of which rails should be moveable is partly dictated by the flexibility of the pick and place machine of the manufacturing process. Many commercially available pick and place machines are provided with a first and third rail fixed (non-movable), or a first and fourth rail fixed. In one embodiment of the present invention, rail 1 may be fixed, while rails 2, 3 and 4 may be moved along the y axis to accommodate variable width boards. Rails 1, 2, 3, and 4 respectively correspond to rails 22, 24, 26 and 28 of FIGS. 6a-6d.

Referring now to FIG. 7, the control of the moveable rails is shown in more detail to include a rail 2 motor 60, and a rail 3 motor 62 each coupled to a front support plate 71, and a rail 4 motor 64 coupled to a rear support plate 73. Each of these motors has independent control over the positioning of the rails relative to rail 1. Each motor is controlled via control logic managed by the computer system of the stencil machine. Each motor moves its associated rail in the y-axis 27 along the spacing rails 70. FIG. 7 also shows more clearly the locations of x-axis motors 66 and 68 that control the advancement of the conveyor belts of the dual track system, as described previously with respect to FIG. 4.

As mentioned above, in one embodiment of the invention, the first rail is fixed. Apertures 69 provide alternative fixed placement locations for the first rail.

In the embodiment shown in FIG. 7, the rails may be adjusted to handle boards ranging in size from 2"×2" to 20"×10" in dual track mode. The system further provides the additional flexibility of being able to operate on larger boards in single track mode. Single track mode may be achieved, as shown in FIG. 8, by moving the rails of the track that are not in use towards the rear support plate 73. Rails 3 and 4 are moved to be adjacent to one another, and in this position will not receive incoming boards. Rail 2 is moved adjacent to Rail 3, and will be used, with rail 1, as usual to support boards to be processed on track A. In one embodiment, in single track mode, the system is capable of processing boards in the range from 2"×2" to 20"×20", depending upon the spacing between rail 2 and rail 1.

Thus, size configurability of the stenciling machine is flexible enough to adapt to spacing requirements if board standards or sizes change. In a preferred embodiment, they may range from two 8.5 inch maximum width boards with a 2 inch space in between the boards to two 10 inch maximum width boards with a 4 inch space between the boards in dual track mode. Similarly, maximum boards widths may range from 17 to 20 inches in single track mode. Variations in the spacing standards is preferably provided in 0.5 inch increments, creating a possibility of 20 rails spacing varieties, but these standards may be altered to accommodate the needs of the user.

The position of the patterns on the stencils used in the moveable rail system are directly dependent upon the specific spacing of the rails for that process. Thus dedicated stencils may be required to support each possible rail spacing option.

In addition to providing a number of stencils to support various size boards and dual and single track modes of operations, a number of porting plates may be provided to ensure that the apertures in the porting plates are appropriately located to provide optimum retention of different size circuit boards.

In one version of the second embodiment, the computer system 21 is configured upon reset or reboot to determine whether the rails are positioned for single track mode or dual track mode. Although this procedure could be accomplished manually, it is desirable to perform this using the computer system as described below to prevent any damage to the stencil machine from a faulty input by a user. The procedure by which the computer control system 21 accomplishes the reset/reboot will described with reference to FIGS. 9-11. FIG. 9 shows the dual rail system of FIG. 7 with the rails positioned in a single track mode with a single worknest 154 positioned between the first rail 22 and the second rail 24. FIG. 10 shows the dual rail system of FIG. 7 with two worknests 150 and 152. Worknest 150 is positioned between the first rail 22 and the second rail 24, and worknest 152 is positioned between the third rail 26 and the fourth rail 28.

FIG. 11 shows a flow diagram of the reset/reboot process 200. The rails in FIGS. 10 and 11 can move in a direction towards the + sign (defined as the positive direction) and can move in a direction towards the - sign (defined as the negative direction).

As shown in FIGS. 9 and 10, each of the second, third and fourth rails has a home sensor 156, 158 and 160. The home sensors may be implemented using optical, magnetic or other available sensors. These home sensors detect whether the sensors are in alignment with one of flags A, B and C. When a sensor detects one of the flags, the sensor is defined to be in a "closed" position. When a sensor does not detect a flag, it is defined to be in an "open" position. By moving the rails and detecting the transitions from open to closed, there are five positions at which the absolute locations of the rails can be determined (note that rail 4 cannot move past flag C in the direction indicated by the + sign).

The rails also include proximity sensors. Rail 4 has proximity sensors 162 and 164, rail 3 has proximity sensors 166 and 168, and rail 2 has proximity sensor 170. The proximity sensors are implemented using optical, magnetic or other available sensors, and are used to detect when the rail is in close proximity to another object (i.e one of the worknests or another rail). Each of the home sensors and the proximity sensors is coupled to the computer system to relay their status (open or closed) to the computer system.

The process 200 for determining the mode of operation upon reset or reboot will now be described with reference to FIG. 11. In step 202, the computer system checks the status of the home sensor 160 on rail 4. If the status of the sensor is "closed", then the process proceeds with step 206. If the status of the sensor is "open", then the process proceeds to step 204 where rail 4 is moved in the positive direction until the status of the home sensor 160 changes to closed (the position of rail 4 in FIG. 10). The process then continues with step 206.

In step 206, rail 3 is moved in the positive direction until either sensor 168 on rail 3 is closed by work nest 152 or until sensor 162 on rail 4 is closed by rail 3. In step 208 the status of home sensor 158 is checked by the computer system. If home sensor 158 is "open," then rail 3 is in the position shown in FIG. 9, and the system is configured for single track mode of operation. The process then proceeds in step 210 to determine the true locations of rails 2 and 3 by detecting the rear edges of flags A and B and then positioning the rails according to default single track mode settings. If home sensor 158 is in the "closed" position at step 208, then rail 3 is in the position shown in FIG. 10, and the system is configured for dual track mode. The process then proceeds in step 212 to determine the true locations of rails 2 and 3 by detecting the front edges of flags A and B and then positioning the rails according to default dual track mode settings stored in the computer system.

The automated reset/reboot process described above offers the advantage of automatically positioning the rails to default single track mode or default dual track mode settings based on the configuration (dual mode or single mode) of the stencil machine without operator intervention.

Referring now to FIG. 12, a third embodiment of the dual rail system is shown, wherein the rails are shortened. In this embodiment the rails have been shortened because, it may occur that a customer, purchasing the dual rail system, may already possess equipment with queuing stations for loading and unloading the stencil machine. Alternatively, it may occur that the pick and place machine that exists at a circuit manufacturing plant already has associated therewith a loader unit. In order to facilitate a variety of existing configurations, this embodiment of the dual rail system is provided with shortened rails 80, 82, 84 and 86. The operation of this unit is similar to that described with regard to FIGS. 6a-6d, with position 1 and position 3 representing respective queue positions in the queuing stations 77 and 79. However, it should be noted that in this embodiment, the boards are moved only to one position within the dual rail system, that position being the work nest 55. Thus this configuration would not have the constraints that may cause system down situations, described above with reference to FIGS. 6a-6d.

Referring now to FIG. 13, a fourth embodiment of the present invention is shown that illustrates the abilities of the dual track system for handling delicate materials, for example green tape. Green tape is a ceramic type substance that is non-rigid before firing. Because of its non-rigid nature, it is usually transported on a pallet through the stenciling system, which provides added support. In FIG. 13, a loader/unloader device 92 is shown coupled to a first set of rails 94 and 96 and a second set of rails 98 and 99. The pallets enter the stencil machine 90 from the loader/unloader on the first set of rails, and return to the loader from the stencil machine on the second set of rails. Pallets are passed from the first set of rails to the second set of rails using a shuttle device 101. The shuttle device receives pallets after printing has occurred from the first set of rails and transports the pallets to the second set of rails. In FIG. 13, the shuttle device is shown in the position to receive a pallet from track A. After receiving a pallet, the shuttle device moves the pallet in the direction of the arrow to provide the pallet to track B.

To accommodate the above-described recirculating rail system, the work nest 55 is altered so that it works only on boards on track A and not on track B. Thus, during operation, when a board such as board A2 is raised in the work nest on the Z-tower, board A1 may still travel unaffected on track B, and return to the loader/unloader 92.

Referring now to FIG. 14, a fifth embodiment of the invention is shown. In the fifth embodiment, a stencil machine 100 has a first set of rails 108 and 110 and a second set of rails 104 and 106. Similar to the fourth embodiment described above, in the fifth embodiment, boards (or some other substrate) are loaded and unloaded into the stencil machine from only one side. The fifth embodiment differs from the fourth embodiment in that boards are loaded and unloaded by the loader/unloader 102 from each of tracks A and B, and printing occurs on both of tracks A and B. In the fifth embodiment, printing occurs on only one of tracks A and B at a time in an alternating manner.

Referring now to FIG. 15, a sixth embodiment of the invention is shown that allows for increased stenciling throughput by permitting the stenciling of two boards with one stroke of the squeegee head 32 and one z-axis motion. The sixth embodiment is similar to the fifth embodiment, except that in the sixth embodiment, a stencil machine 120 includes a mechanical fixturing device 112 used in place of an optical alignment system to properly align the boards in the x, y, and θ axes. The mechanical fixturing device is prealigned with the stencil before the boards are received by the stencil machine, and supports the boards in the proper aligned position. In one embodiment, the mechanical fixturing device uses side snuggers for holding the board in proper alignment in the stencil machine. This embodiment is preferably used with boards or other substrates for which alignment of the board can be achieved using the edges of the board.

The mechanical fixturing device eliminates the need for the optical alignment step described with reference to the stencil machine of FIG. 1. In the embodiment of FIG. 15, a loader/unloader 102, or some other type of loader is used to forward the boards to the work nest. The squeegee is positioned at point A and applies the solder paste over the stencil in a manner as 25 described above, thereby printing both boards. The boards are then retrieved by the loader/unloader.

The mechanical fixturing device 112 of FIG. 15 may also be used in the embodiments of the present invention that are designed to receive material on one side of the stencil machine and deliver material on another side of the stencil machine to, for example, a pick and place machine, thereby increasing throughput of the stencil machine. In this situation, the pick and place machine should be configured to receive and operate on two boards simultaneously to maximize throughput.

Multiple embodiments of a dual track system have been described to show the flexibility and potential of such a system to many different circuit board manufacturing requirements. It should be noted that, although the above description discusses a dual track system, the present invention is not limited to such a construction. It is envisioned that a multi-track system, having more than two tracks, could also be provided by adjusting the ordering sequence of board processing, and potentially adding more hardware if feasible to maintain performance. Note that the multi-track system could comprise a number of fixed tracks or similarly use the moveable rail mode of operation taught herein.

Accordingly, a method and apparatus has been shown for providing increased performance in the stencil phase of the circuit board manufacturing process. By providing a dual track system, circuit board throughput is increased, and dwell time between machines is reduced. In addition a processing sequence is provided that allows for existing stenciling components to be used, thus minimizing the extent of additional hardware and minimizing the overall cost of the system. In addition, according to another embodiment of the invention, the rails of the dual tracks are moveable to allow for the processing of different width boards by one stencil machine. In addition, the moveable rails allow for increased compatibility between the stenciling machine and a variety of pick and place machines provided by other vendors and varying Printed Circuit Board (PCB) sizes. Further, the use of moveable rails allows the stenciling machine to be converted for operation in single track mode when desired. The dual track rails may alternatively be shortened, to accommodate existing equipment in the circuit board manufacturing process. In addition, it has been shown how the throughput for handling delicate materials may be accomplished. Finally, an apparatus that provides high throughput by allowing for printing on both rails has also been described.

Although embodiments of the invention have been described primarily with respect to stencil machines that print solder paste on circuit boards, the embodiments present invention is not limited to solder paste printers, but may be used in other printers for printing materials such as glues and inks on substrates other than circuit boards.

Having described a preferred embodiment of the invention, it will now become apparent to one of skill in the art that other embodiments incorporating its concepts may be used. It is felt, therefore, that this invention should not be limited to the disclosed embodiment, but rather should be limited only by the spirit and scope of the appended claims. 

What is claimed is:
 1. A stenciling system for stenciling circuit boards comprising:two tracks, each of said tracks comprising a set of rails, each rail of each set having a conveyor belt for supporting said circuit board during advancement through said stenciling system, each of said tracks having an independently controlled conveyor motor, each of said tracks being configured to advance said circuit boards to at least one predetermined position in said stenciling system, including a stencil position; a tower, disposed beneath said tracks, for raising said circuit boards in said stenciling position off of said conveyor belts of said tracks; and a stencil disposed above said tower, said stencil having two patterns, each one of said patterns aligning with one of said stenciling positions of said tracks.
 2. The stenciling system of claim 1, further comprising:a material dispensing unit disposed above said stencil, for dispensing a material on said stencil and dispersing said material over one of said patterns.
 3. The stenciling system according to claim 2, wherein said material is selected from a group of materials including solder, glue, and ink.
 4. The stenciling system of claim 2, further comprising:a vacuum system comprising two hoses, and two chucks, each chuck for coupling one of said hoses to one of said tracks at said stenciling position, said vacuum system for retaining a circuit board in said stencil position.
 5. A dual track stenciling machine comprising:a pair of tracks, each track having a pair of rails for forwarding a circuit board to a process area of the stencil machine for processing, wherein both tracks are controlled by one device; a work nest forming the process area in the center of the tracks; a stencil support, disposed within the process area, that supports a stencil in the stenciling machine; and a squeegee head, disposed above the stencil support, for disposing material through the stencil onto said boards.
 6. A stenciling system for use in circuit board manufacturing, comprising:a frame that supports components of the stenciling system; a plurality of sets of rails, coupled to the frame, each of said plurality of sets of rails for supporting and moving a plurality of circuit boards in one direction trough said stenciling system, wherein each rail of said plurality is separately controlled to be moveable in a direction orthogonal to said one direction.
 7. The stenciling system as recited in claim 6, wherein said stenciling system is configured to operate in either multi-track mode, wherein all of said plurality of tracks are used for stenciling circuit boards, or in single track mode, wherein only one of said plurality of tracks is used for stenciling circuit boards.
 8. The stenciling system according to claim 6, wherein the advancement of each of said rails along said spacing tracks is controlled by a programmable controller. 